Clean Architectures Precursors

The precursor of layered architectures is the classic MVC pattern from Smalltalk. This disentangles the model from the user interface (controller and view), so that the model does not depend on the UI.

More complex systems do not have just one user interface, but possibly multiple user interfaces (such as a web app or a mobile app). MVC isolates the business logic on the server from these UIs, so the server doesn’t care about which kind of app accesses it.

Typically, the server still depends on backend services such as databases or third party providers. This is perfectly fine, and leads to a simple layered architecture.

The Hexagonal Architecture goes further and stops making a distinction between frontend and backend. Any external system might be an input (data source) or an output. Our core system defines the necessary interface (ports), and we create adapters for any external systems.

One classic approach for strong decoupling is a Service Oriented Architecture (SOA), where all serices publich events to and consume events from a shared bus. A similar approach was later popularized by microservices.

Clean Architecture could be considered an evolution of related approaches like the Onion Architecture by Jeffrey Palermo (2008) and the Hexagonal Architecture (“Ports and Adapters”) by Alistar Cockburn and others (< 2008).

Different problems have different requirements. Clean Architecture and related approaches favor decoupling, flexibility, and dependency inversion, but sacrifice simplicity. This is not always a good deal.